Not Applicable
Not Applicable
1. Field of the Invention
In general, the electrical device to which the present invention relates belongs to the xe2x80x9cliquid electrolytic capacitorxe2x80x9d category. Technical jargon amongst chemical engineers recently, for example at the Third Hawaii Battery Conference, manifests their preference to refer to that category as xe2x80x98electrochemical capacitorsxe2x80x99. Subgrouping of electrochemical capacitors is based on mode of storing electrical charges, and includes xe2x80x98electrical double-layer capacitorsxe2x80x99 (EDLC) and xe2x80x98pseudo-capacitorsxe2x80x99 (PC). Electrode materials commonly utilized are high surface area carbons for EDLC, and transition metal oxides such as hydrous ruthenium oxide for PC.
To define what xe2x80x98supercapacitorxe2x80x99 means in the TITLE above and hereinafter, the termxe2x80x94sometimes encountered in print with a space between xe2x80x98superxe2x80x99 and xe2x80x98capacitorxe2x80x99xe2x80x94refers to a composite type electric charges storing device combining features of the aforesaid two subgroups of electrochemical capacitors, EDLC and PC, that owes its capacitance partly to charge storage in electrical double layers formed at the phase boundary between electrode and electrolyte, and partly to a transient change of oxidation state in a pseudo-capacitance material like the ruthenium oxide already mentioned.
One factor motivating attempts to devise superior supercapacitors is the prospect of using them as power sources for self-propelled electric vehicles. Today""s leading edge supercapacitors can store about one-fourth as much energy for a given volume and weight as a lead acid battery. If compared only in terms of weight and size needed to store a given amount of energy, therefore, lead acid batteries are considerably lighter weight and more compact than the best super-capacitors. However, the ability of supercapacitors to release substantially all their stored energy within a very short period, say five to ten seconds, is unmatchable by batteries. Thus are supercapacitors especially fitting to contemplate for onboard auxilliary power supply in electric vehicles, as an energy source procuring, on demand, the fast accelerating performance that most electric vehicles based solely on power from lead acid batteries lack. By suitably incorporating supercapacitors into the power system of electric vehicles that use lead acid or any other secondary batteries for basic cruising power, the results would be to improve acceleration for passing in traffic, and to achieve fast starts reaching say 96 kmh (60 mph) in under ten seconds.
Both secondary batteries, and supercapacitors in a complementary role to the batteries in an electric vehicle powering system, would be rechargeable by d.c. power input to them from any known system already devised to recharge the batteries. A petrol-fueled combustion engine driving a d.c. generator can be aboard the vehicle. Hybrid vehicle proposals combining fuel cells and secondary batteries are known to suggest recharging batteries with fuel cell generated power, in that case utilizing the batteries in a kick-in-when-needed type auxilliary role to fuel cell power for the basic cruising. There would be no obstacle to charging capacitors the same way. Another highly pertinent system in this context is the current-producing type of regenerative braking system that recovers energy of braking and converts it to d.c. current for storage. Usually that proposal is directed to recharging batteries, but again the recharged item could as well be a supercapacitor. The present invention is considered particularly applicable to supercapacitors used as an auxilliary source of power in an electric vehicle equipped for recharging of both batteries and capacitors during normal road travel operation, via either combustion engine-driven current generation, fuel cell power generation, or current-producing regenerative braking.
2. Description of Related Art
The leading edge in the art of devising superior supercapacitors has encountered very recently, a laboratory-verified difficulty with finding an effective balance between a highly desirable regularly interconnected pores structure for carbon electrodes procuring electrical double-layer capacitance, on the one hand, and the use of pseudocapacitance material to load the pores, to raise specific capacitance by adding pseudo-capacitance, on the other hand. To gain appreciation of this trade-off problem, one may first turn to a published comparison between a typical molecular-sieving carbon body and a new porous carbon body with a regularly interconnected network of somewhat larger pores than in the molecular-sieving carbon. If electrochemists of the past have sometimes tended to too blithely assume that larger surface area for electrodes is always better, the report next briefly reviewed may give pause for re-assessment of the suitability of porous materials containing myriad randomly distributed very small pores.
Seoul National University researchers S. Yoon et al, in their report, xe2x80x9cElectrical Double-Layer Capacitor Performance of a New Mesoporous Carbonxe2x80x9d, Journal of the Electrochemical Society, 147 (7), pages 2507-2512 (2000), examine the differences between their new mesoporous carbon body and a typical molecular sieving carbon body, wherein the latter possesses the larger surface area of the two bodies because of smaller pores, and yet is outperformed in terms of charging/discharging rate capability at higher current densities by the new carbon body having smaller surface area because of larger pores, but featuring a regularly interconnected pores network. Even though the mesoporous body calculates as being of lower specific capacitance, in fact it stores more charge at high current density than does the molecular sieving carbon with higher calculated specific capacitance. S. Yoon et al explain the differences largely in terms of how phenomena are dominated by electrolytic resistance effects within pores. An ionic motions problem is involved. To further understand implications highly pertinent to the present invention, a follow-up report to the foregoing report is brought into the picture, bearing in mind that both types of carbon electrodes in the first report are electrical double-layer capacitors only, neither of them having a pseudo-capacitance aspect.
A report presented at the Third Hawaii Battery Conference by Seoul National University researchers J. Jang et al, entitled xe2x80x9cElectrochemical Capacitor Performance of Ruthenium Oxide/Mesoporous Carbon Electrodesxe2x80x9d, is concerned with combining pseudo-capacitance procuring electrode material with electrical double-layer capacitance procuring electrode materials the latter being the same new mesoporous carbon as for the superior electrodes of the preceding report by S. Yoon et al, colleagues at Seoul with J. Jang et al. Ruthenium oxide loading of pores of the mesoporous carbon adds pseudo-capacitance related characteristics.
While success at raising the specific capacitance under relatively lower current densities was achieved by loading the carbon mesopores with ruthenium oxide, the researchers found evidence of pore-blocking that shifts the problem area back again to impaired ionic motions at higher current densities. Quoting J. Jang et al, bottom of page 8 and top of page 9: xe2x80x9cIn this work, we tried to enhance the specific capacitance of mesoporous carbon electrodes by loading ruthenium oxide that carries the pseudo capacitor characteristics. An enhanced specific capacitance is expected with these composite electrodes as two types of capacitors are combined. This beneficial effect may, however, be counterbalanced by a loss of rate capability because an excessive ruthenium oxide loading inside the mesopores may narrow down the pore size that eventually retards ionic motions.xe2x80x9d (emphasis added)
Drawing on the published revelations of the Korean research, the present inventor formulates the following desideratum: that in order to optimize high current performance of supercapacitors intended as compact energy reservoirs in electric vehicles, something is required to be done to obviate the recently evident trade-off between high specific capacitance and fast charging/discharging rates at high current, so that these desirable features are both optimized. With a few exceptions addressed further on, nothing remotely like what is to be done in accordance with the present invention existed in the art a year ago as a previous suggestion that may be routinely practiced. Two of the same inventor""s patents require to be considered, before getting at the nature of the exceptions.
Robert N. O""Brien and Kalathur S. V. Santhanam in U.S. Pat. No. 5,051,157 issued Sep. 24 1991, for a xe2x80x9cSPACER FOR AN ELECTROCHEMICAL APPARATUSxe2x80x9d (assignee: University of Victoria) disclosed a method combining applied magnetic fields and specially shaped spacing structure placed between opposed electrodes, for optimizing stirring action in density-driven, naturally convecting, bulk liquid electrolyte circulating about the space between opposed electrodes where the new structure should be placed. The invention works whether the electrodes happen to be in a: (1) lead acid battery; (2) electromachining device; or, (3) chlorine gas producing saltwater electrolyzer. No capacitors are mentioned. This patent teaches more, however, than just the design of the specially slitted spacer having magnetized material embedding therein. Also disclosed are: that the apparatus casing should be xe2x80x9cformed of silicon steel to provide a good magnetic environmentxe2x80x9d, that the casing top be crossed by straps of the same material xe2x80x9cto complete the magnetic circuitxe2x80x9d, that the casings be internally lined by xe2x80x9cpolyethylene to avoid corrosionxe2x80x9d, and that such a casing xe2x80x9cmay also be made into a permanent magnet, by incorporation of a magnetic material on, or by fusing of a magnetic sheet to the casingxe2x80x9d. (emphasis added)
The underlined suggestion above is among a limited number of exceptions to a general consideration by the present inventor that not very much published to date in the area of magnetically enhanced electrochemical cells technology is of material relevance to the present invention related to improving supercapacitor. It is considered that the bulk liquid stirring mechanism for reduction of internal cell resistance suggested in the O""Brien/Santhanam spacer patent would be expected by anyone following that main suggestion to only work in such apparatuses as (1)-(3) above, which typically have a significant quantity of low viscosity liquid electrolyte between electrodes that is known to manifest density-driven convection even during normal operation. In other words, the O""Brien/Santhanam patent holds out no promise of utility of its main suggestion unless it is directed to appropriately convective settings, so to speak.
Capacitors would, more likely than not, not come to the mind of an informed person absorbing information from U.S. Pat. No. 5,051,157.
Between issuance of the U. Vic. patent for the co-invented spacer, and filing the provisional application corresponding to this disclosure, the present inventor kept active in the growing field of magnetoelectrolysis. Others have cited his laser interferometric investigations as significantly contributing, as mentioned for example in the overview article entitled xe2x80x9cApplications of Magnetoelectrolysisxe2x80x9d, by R. A. Tacken and L. J. J. Jansen, Journal of Applied Electrochemistry, 25, 1 (1995).
The technology of secondary batteries has burgeoned of late but applied magnetic fields have not generally been adopted as a panacea, and wherever a success for magnetic enhancement is won, it seems so far always to depend on non-routine tailoring of magnetic field generating means to fit highly differentiable battery designs. A few basic configurations were tackled in the inventor""s U.S. Pat. No. 6,194,093 B1 entitled xe2x80x9cMAGNETIZED CURRENT COLLECTORS COMBINED WITH MAGNETIC SHIELDING MEANSxe2x80x9d. Its Feb. 27, 2001 issuance date is three weeks after filing the provisional application corresponding to the present disclosure.
As usual, more was taught by publication of the patent than just the gist of the invention. Although most of the disclosure concerned novel folded-over sheet magnets encased by active electrode material and in turn themselves encasing inserts of magnetic shielding material, the patent also discloses much simpler structures that have neither encasements or inserts. The simpler structures were by text and figures taught to be useful at the extreme left or right ends of certain batteries, and can be single-sided magnetized electrodes mounted directly against a casing wall. Among the xe2x80x9cfunctions not really concerned with magnetization of current collectorsxe2x80x9d it was mentioned without detail of description that there could be xe2x80x9cmeans for replacement of removable negative electrodesxe2x80x9d. (emphasis added)
Picking out the above emphasized suggestion of something removable and replaceable, from U.S. Pat. No. 6,194,093 B1, is another instance of the relatively rare teachings in known prior art that the present inventor does consider to be of possible material relevance to at least limited aspects of the invention described below.
The recent current collectors patent also contains indications of a focus diverging away from the former focus of the spacer patent on the region of bulk liquid convection between electrodes. Now, in U.S. Pat. No. 6,194,093 B1, there is more attention directed to xe2x80x9cregions between asperities and in pores of electrodesxe2x80x9d. (emphasis added)
Duty of candour obliges acknowledging the logical connectibility of doing something about ionic motions in pores of electrodesxe2x80x94as taught in U.S. Pat. No. 6,194,093 B1 first published on Feb. 27 2001xe2x80x94and what is presently proposed to do for satisfying the above set forth desideratum in the new context of supercapacitors. That the connection is logical enough to be possibly made does not mean the connection would probably be made by anyone.
Secondary batteries and related electrolysis devices, on the one hand, and capacitors, on the other hand, manifest significant differences between them that are not discountable customarily by technologists. The differences are considered by the present inventor to be substantial enough that technologists would not, by the time of original disclosure of the present invention, have adopted as a routine matter the transferring over to supercapacitors technology of magnetic enhancement methods specified for highly specific types of secondary batteries.
An important point of commonality between the different endeavors, viz., that in this inventor""s recently patented magnetized current collector invention, on the one hand, and that of meeting the aforesaid desideratum concerning supercapacitors, on the other hand, is that engaging either set of issues always involves attending to the electrical double-layer formation at the phase boundary between electrode and electrolyte. Yet, even so, no secondary battery known to the inventor, that has yet been proposed for magnetic enhancement, has had the unique characteristic of supercapacitors, of combining electrical double-layer capacitance with pseudo-capacitance in the manner of the above reported Korean experiments. Had there been such a supercapacitor-like battery, obviously, its analogousness would make the presently proposed subjection of materials in a supercapacitor to influence of a magnetic field considerably less unheralded than the factual case here.
Three briefly indicated minor points of suggestion from certain references that were cited in the background of the magnetized current collectors patent will assist in concluding this background discussion.
First, J. Von Brimer in U.S. Pat. No. 3,597,278 (Aug. 3, 1971) taught that the magnets his drawing shows between lead-acid battery electrode plates xe2x80x9ccan be positoned outside of the plates, inside or outside of the cellxe2x80x9d. (emphasis added) This generously gave technologists lots of options, to supplement what guidance in so fresh an application was available to lead-acid battery makers in 1971. It remains true to this day, however, that magnets may be placed outside of a cell and yet affect events inside.
Second, Takahashi et al in U.S. Pat. No. 4,000,004 (Dec. 28, 1976) taught about their magnetized iron-electrode that xe2x80x9cafter its magnetism is weakened through repeated charging and discharging, the anode can again be magnetized easily without having to disassembling the battery constructionxe2x80x9d. It would be helpful to know whether the magnetizing device (not shown) is to be carried to where the battery is when the part loses its magnetism, or whether the whole non-disassemblable battery is to be removed from whatever device it powers, then being carried away to some magnetizing installation.
Third, Kawakami et al in U.S. Pat. No. 5,728,482 (Mar. 17, 1998) taught a preference for using xe2x80x9ca magnetic material exhibiting little degradation due to oxidation during the charging/discharging cycle of the batteryxe2x80x9d. This policy is wise and should be adopted by everyone aware of possible chemically-induced demagnetization.
Clearly a variety of contributors to the endeavor of improving batteries by inclusion of magnetic field producing means have made a host of technical suggestions generally like the three above. The whole repertoire can be drawn on by creative minds for inspiration of inventing new supercapacitors improvements, but only after it is at first suggested somewhere that magnetically enhanced supercapacitors are desirable arid feasible.
To summarize, this invention consists of magnetizing the metal backing parts of otherwise generally conventional supercapacitors, positioning at least one magnetized part per capacitor adjacent a porous electrode in the capacitor that is of composite construction using two materials, one for procuring electrical double-layer capacitance, and one for procuring pseudo-capacitance. The purpose of the magnetized part is to produce enhanced magnetohydrodynamic stirring both in a region of bulk liquid electrolyte adjacent entry positions to a network of interconnected pores of the composite electrode, and also inside the pores themselves, in order to remedy the reported deficiency of ionic motions that precludes an optimum balance of high specific capacitance and high rate capability for large current charging and discharging. The invention works by obtaining less internal resistance.
The magnetized metal backing parts require being located somewhat differently for differently shaped supercapacitors, ie., depending on the basic supercapacitor device geometry, because the magnetized part location must always be, as the invention claims, adjacent an electrode formed as a composite type utilizing both electrical double-layer procuring material and pseudo-capacitance procuring material in the usual manner of loading the latter material into pores in the former. For embodiments wherein the composite structure of combined electrode materials coats the interior surface of a non-magnetized casing, a preferred location for the adjacent magnetized backing part is against the casing, on the opposite surface thereof from the electrode forming materials, assuming the non-magnetic casing, when used, to be of relatively thin sheet material lacking any significant effectiveness as magnetic shielding. A portion of the casing itself, however, may be the magnetized backing part, if desired, in which version then separate magnetized parts are not needed for application against the casing exterior.
The below illustrated embodiments that are to be described in detail do not exhaust configurational possibilities for the invention.
The rate capability to be improved as already stated pertains to the heart of the xe2x80x98counterbalancingxe2x80x99 problem uncovered by the second cited report above, from Seoul National University researchers. Retaining the desirable feature of loading pores of a capacitor electrode with as much pseudocapacitance material as practicable to get high specific capacitance, while also obtaining shorter times of full charging/discharging, by deploying a magnetized metal backing part in the manner described is exclusively the suggestion of the present inventor, who believes that nobody else has perceived both the desirability and feasibility of doing what he proposes.
For needed guidance of the technologists, not to be left out of account are the two states of a supercapacitor when it is neither being charged or discharged; after full charging and before discharging; and after discharging and before recharging. Retention of the magnetic field of the proposedly magnetized metal backing parts is neither detrimental or essential for these particular two states of the device. Any notion that supercapacitor charging and discharging processes would be affected at all by stirring the normally stagnant electrolyte of an already fully charged or already fully discharged supercapacitor is outside the concept of the invention. Since no stirring is needed at those certain times, neither is presence of the magnetized parts.
Accordingly, for one embodiment of the invention there is provided integral means for removing magnetized metal backing parts at appropriate times. In certain instances to be described in detail below, the removal may entail partial electrode assembly removal, inasmuch as electrode assemblies of multi-element structure are possible, in which case a part only of electrode assembly structure, exemplified for two-sided electrode assemblies as core or insert, may be removed. For embodiments with this feature, the removable insert is the magnetized metal backing part.
The measures suggested for removability of parts will procure a number of significant advantages over and above and in aid of the main purpose. Diagnosis of extent of magnetization of the parts, checking them for possible need for re-magnetization on a scheduled basis is facilitated.
However, embodiments lacking the removal means need not be degraded from their usual capabilities and functions for a reasonable product lifetime, because such embodiments (without the removal means) may be built using higher quality permanent magnet materials for their magnetized parts. The parts in both non-removable and removable versions should be made of materials that are pre-graded for known extent of retention of magnetism under the conditions of the duty expected, specifically the magnitude of current handled by the modified supercapacitor. It is foreseeable that the cumulative effect of a large amount of use with sudden high current surges of electricity immediately adjacent the magnetized parts could tend to demagnetize them, more so if made of inferior grade material, by which is meant; a grade more liable to be demagnetized than better grades. The most careless selection, of course, would be to choose so inferior a grade of magnetic material that parts in the supercapacitor made of it would lose their magnetization at the first instance of discharge. At the other end of materials selection, it appears likely that magnetic materials producers will derive from publication of the present invention an impetus to develop materials specially tailored to retain magnetism at locations immediately adjacent structures passing large sudden surges of current. Probably, the best possible material for this service has not yet been preparedxe2x80x94which is not to say the invention cannot be carried out in an effective manner that leaves some room for improvement respecting materials
It is because current-associated magnetic fields accompanying high current sudden discharges from capacitors are significant, and are in fact associated in other applications with design of means for switching off (demagnetizing) permanent magnets, for example in magnetic holding devices, that the technologists assigned manufacturing of magnetized metal backing parts for use in supercapacitors in accordance with this invention shall properly heed demagnetization properties of materials used.
Other objects, advantages, and novel features of the present invention will become apparent from the following detailed description when considered in conjunction with the figures of the accompanying drawing next briefly described.